All or most of these applications concern non-real-time messaging (data collection or short text messaging), wherein a great number of user terminals transmits short messages with a very low duty-cycle. Typically, individual messages have a length of a few tens to a few hundreds of bytes, and a low bit rate (e.g. a few kbps to a few tens of kbps). The delivery delay should be from a few seconds to a few minutes (even more if the terminal is not in visibility of the satellite). The typical activity factor is estimated in a few tens of Kbytes per user per day (e.g. 100 messages of 100 bytes=10 KB), i.e. a very low one.
Such a low duty-cycle traffic makes efficient implementation of the return link (or uplink) challenging, because:                Classical Demand Assignment Multiple Access—DAMA or Contention Free DAMA do not work properly with this type of traffic characterized by large number of users with unpredictable low duty-cycle traffic patterns;        closed loops for timing synchronization as required for slotted random access systems such as Slotted-Aloha or the more recently proposed Contention Resolution Diversity Slotted Aloha (CRDSA)—see document EP 1 686 746 would require an unacceptable signalling overhead,        power control as required for spread Aloha random access system would require an unacceptable signalling overhead.        
The Spread-Spectrum Aloha (also known as “Spread Aloha”) protocol—SSA—described in the paper by O. del Rio Herrero et al. “Spread-spectrum techniques for the provision of packet access on the reverse link of next-generation broadband multimedia satellite systems”, IEEE Journal on Sel. Areas in Comm., vol. 22, no. 3, pp. 574-583, April 2004, shows potentially interesting features. It provides a higher throughput capability than CRDSA for the same Packet Loss Ratio target under equal power multiple access conditions and using powerful physical layer FEC (Forward Error Correction), i.e. of the order of G=0.45 b/s/Hz for a packet loss ratio of 10−3). Furthermore SSA allows operating in a truly asynchronous mode, i.e. without the need of synchronizing the terminals to ensure “slotted” operation. The basic principle of the Spread-Aloha scheme is the following: when a user terminal has a packet to transmit, it picks up at random one spreading sequence among a predetermined set of sequences, and one possible spreading code phase, and transmits it (a single spreading sequence may be sufficient in some applications). If two messages, transmitted using a same spreading sequence and spreading code phase, collide and are lost, transmission is tried again after a random delay. One of the major weakness of SSA is it fragility to packet power unbalance conditions which is heavily curtailing its performance. In a random access satellite network it is very difficult to achieve tight power control thus SSA practically achievable efficiency is very modest.
Document EP 2 159 926 describes an improvement of SSA (called E-SSA, for Enhanced Spread Spectrum Aloha), using Iterative Successive Interference Cancellation to recover corrupted packets, thus increasing the throughput of the channel in particular when received packet power unbalance occurs. Contrarily to SSA, the E-SSA detection process allows to achieve higher throughput in the presence of unbalanced packets power. Document EP 2 159 926 also discloses a basic decentralized transmission control algorithm (SDUPTC: SNIR-Driven Uplink Packet Transmission Control). Its principle is simple: user terminals only transmit when the downlink signal quality is good i.e. the signal strength or better signal-to-noise plus interference ratio (SNIR) is within a certain window representative of line of sight conditions (LOS). If this is not the case the transmission is delayed until LOS conditions are verified. A simple congestion control mechanism is also disclosed, reducing the transmission rate when the channel is congested.